1. Field of the Invention
The present invention relates to an image forming apparatus having a sheet feeding apparatus for feeding sheets after the air was blown to the sheets and the sheets were loosened.
2. Description of the Related Art
Among conventional image forming apparatuses such as copying apparatus, printer, and the like, there is an apparatus having a sheet feeding apparatus for separating sheets stacked on a tray from the top sheet one by one and feeding them to an image forming unit.
As such a sheet feeding apparatus, as disclosed in Japanese Patent Application Laid-Open No. H07-196187, there is an apparatus of a type in which the air is blown to a sheet bundle stacked on a tray, a plurality of sheets is floated and separated, and thereafter, the sheet is adsorbed to an adsorbing and conveying belt and fed.
FIG. 20 is a cross sectional view illustrating an example of a sheet feeding apparatus with such a construction.
In FIG. 20, a container 11 is provided for a main body of an image forming apparatus (not shown) so that is can be freely pulled out. Sheets S are enclosed in the container 11. The container 11 has: a tray 12 which can be freely elevated upward and downward and on which a plurality of sheets S is stacked; and a rear edge regulating plate 13 for regulating a position of a rear edge as an edge on the upstream side in the sheet feeding direction of the stacked sheets S. Further, the container 11 has side edge regulating plates 14 and 16 each for regulating a position of a side edge as an edge in the width direction which perpendicularly crosses the sheet feeding direction of the stacked sheets S. A slide rail 15 which is used to pull out the container 11 from the image forming apparatus main body (not shown) is provided between the image forming apparatus main body and the container 11.
In FIG. 20, an adsorbing and conveying belt 21 adsorbs the sheet and feeds it. An adsorbing fan 36 allows the sheet S to be adsorbed to the adsorbing and conveying belt 21. An air blowing portion 30 blows the air to a front edge of an edge portion on the downstream side in the sheet feeding direction of a sheet bundle SA. The air blowing portion 30 has a separating fan 31, a separating duct 32, a loosening nozzle 33, and a separating nozzle 34. The air blown out by the separating fan 31 is blown toward the sheets from the loosening nozzle 33 and the separating nozzle 34 through the separating duct 32.
In the sheet feeding apparatus with such a construction, when the user pulls out the container 11, sets the sheets S therein, and thereafter, stores the container 11, the tray 12 is lifted up by a driving unit (not shown) in the direction shown by an arrow A in FIG. 21. The tray 12 is stopped at a position where a distance between an upper surface of the sheet bundle SA and the adsorbing and conveying belt 21 is equal to B and, thereafter, prepares for a feeding signal.
Subsequently, when the feeding signal is input, the separating fan 31 of the air blowing portion 30 is made operative and sucks the air in the direction shown by arrows C in FIG. 22. The air is blown toward the front edge surface of the sheet bundle SA from the loosening nozzle 33 and the separating nozzle 34 through the separating duct 32 in the direction shown by arrows D and E, respectively. Thus, a few upper sheets Sa in the sheet bundle SA float and are loosened. On the other hand, the adsorbing fan 36 is made operative and blows the air in the direction shown by arrows F in FIG. 22. At this time, an adsorbing shutter 37 provided for an adsorbing duct 38 is closed.
Auxiliary separating fans 17 and 18 are attached to the side edge regulating plates 14 and 16, respectively. The air from the auxiliary separating fans 17 and 18 is blown to side edges of the sheet bundle SA from openings 14A and 16A. By providing the auxiliary separating fans 17 and 18 as mentioned above, the floating and separation of the sheets Sa are more certainly performed.
Subsequently, when a floating state of a plurality of floating sheets Sa is stabilized after the elapse of a predetermined time after the feeding signal had been input, the adsorbing shutter 37 is rotated in the direction shown by arrows G as shown in FIG. 23. Thus, a sucking force in the direction shown by arrows H is generated by the adsorbing fan 36 from a hole for sucking (not shown) formed in the adsorbing and conveying belt 21. A top sheet Sb in the floating sheets Sa is adsorbed to the adsorbing and conveying belt 21.
By rotating a belt driving roller 41 in the direction shown by arrows J shown in FIG. 25, the sheet is conveyed in the direction shown by an arrow K. Further, the sheet is conveyed to a conveying path on the downstream side by a pulling-out roller pair 42 which is rotated in the direction shown by arrows M and P.
In the sheet feeding apparatus, a degree of ease upon floating of the sheet differs depending on a material (thickness and weight) of the sheet. Therefore, a rotational speed of the separating fan 31 is controlled according to the material of the sheet so as to obtain the optimum floating amount, thereby adjusting a wind pressure of the air which is blown. For example, if the sheet is made of one of a thin material and a light material, the control is made so as to decrease the rotational speed of the separating fan 31. If the sheet is made of one of a thick material and a heavy material, the control is made so as to increase the rotational speed of the separating fan 31. Such a technique has been disclosed in Japanese Patent Application Laid-Open No. 2005-96992.
There has also been proposed such a technique that the material (thickness and weight) of the sheets stacked on the tray 12 is input from an operating unit of the image forming apparatus, thereby controlling so that the air is blown by a predetermined blowing amount according to the input material of the sheet. In recent years, in association with the realization of a color image in the image forming apparatus, what is called coating paper in which the surface of the sheet has been coated with a coating material as a sheet for color printing is often fed. In the case of such coating paper, there is a case where a force (adsorbing force) by which the sheets are mutually adhered is equal to 10N or more depending on a temperature or a humidity of a use environment.
When such coating paper is separated and fed, in the conventional image forming apparatus in which by blowing the air to the sheet bundle, the sheets are separated and fed, there is a case where such a double feeding that the sheets are conveyed in the overlapped state occurs. Further, there is also a case where ten or more sheets are fed in a lump and a paper jam occurs in the conveying path.
To float the heavy and large sheets whose basis weight is equal to or larger than 200 g/m2, even if it is assumed that there is not an influence of the adsorption between the sheets as mentioned above, a very large wind pressure is necessary in order to simply float. Further, for example, in the case of conveying the sheets of the A4 size at a rate of about 70 to 100 sheets per minute, a time which is necessary for the loosening and separation per sheet (that is, a time which is necessary until the sheet is stably floated) becomes short and there is a risk that the sheet cannot be sufficiently loosened.
A compressor, a large turbo fan, a scirocco fan, or the like is often used as an air blowing portion 30 so that the air of a high pressure can be generated. However, they are typically large, heavy, and high in costs. There is, consequently, a risk of causing a large size, a high price, and the like of the apparatus.
On the other hand, for example, in the case where the coating paper of the A3 size of 200 g/m2 are loosened and conveyed at a rate of 50 sheets per minute in an environment of a room temperature of 30° C. and a relative humidity in a range from 60 to 80%, it has been found from experimental results that the air blowing portion 30 needs to have an ability of accomplishing a wind pressure of 650 Pa (pascal).
Among scirocco fans which are used in the sheet feeding apparatuses of the image forming apparatuses such as a copying apparatus which can output the sheets of the A4 size at a rate of about 50 to 70 sheets per minute and the like, a diameter of impeller of the relatively large scirocco fan is within a range about from 80 to 120 mm. According to such a scirocco fan, the air of the pressure which is fairly higher than that of an axial-flow fan having the same diameter is obtained. However, if the impeller having the diameter of, for example, 120 mm is attached to the air blowing portion 30, the wind pressure of up to about 420 Pa is obtained.